1. Field of the Invention
The present invention relates to an ultrasonic transmitting and receiving apparatus to be used for diagnoses of organs in a living body and nondestructive inspections by transmitting and receiving ultrasonic waves.
2. Description of a Related Art
Generally, in an ultrasonic transmitting and receiving apparatus to be used as an ultrasonic diagnostic apparatus and an industrial flaw detecting apparatus, etc., an ultrasonic probe is used which includes plural ultrasonic transducers having functions of transmitting and receiving ultrasonic waves. Image information on an object to be inspected is obtained based on intensity of ultrasonic echoes by scanning the object with an ultrasonic beam formed by combining plural ultrasonic waves and receiving ultrasonic echoes reflected from inside of the object by using such an ultrasonic probe. Further, a two-dimensional or three-dimensional image on the object is reproduced based on the image information.
In the ultrasonic transmitting and receiving apparatus, as a method of transmitting and receiving an ultrasonic beam by using an ultrasonic transducer array formed by plural ultrasonic transducers, the following methods (1) and (2) are known.
(1) Method of Dividing Beams on Reception
FIG. 5A is a schematic diagram for explanation of transmitting an ultrasonic beam according to a conventional method, and FIG. 5B is a schematic diagram for explanation of receiving an ultrasonic beam according to the conventional method. In this method, ultrasonic pulses are intermittently transmitted from respective plural ultrasonic transducers 101 that form an ultrasonic transducer array 100 based on drive signals supplied from plural pulsers provided in a transmission system. Those ultrasonic pulses are transmitted from the ultrasonic transducer array 100 toward an object to be inspected as shown in FIG. 5A, and propagates within the object to form an ultrasonic beam 102.
The ultrasonic beam 102 gradually becomes narrower while it travels in a region at a short distance from the beam transmitting position, becomes narrowest at a focal point F, and gradually becomes broader afterwards. The ultrasonic beam is reflected by a reflector existing within the object so that an ultrasonic echo is generated. As shown in FIG. 5B, the ultrasonic echo is received by the ultrasonic transducer array 100. The plural ultrasonic transducers 101, which form the ultrasonic transducer array 100, output RF signals. Predetermined delays are given to the RF signals and the RF signals are added to each other. Thus the RF signals are subjected to reception beam forming processing by phase matching computing means provided in a reception system, and thereby, reception signals with respect to each ultrasonic beam are obtained. In this example, three reception ultrasonic beams 103, 104, and 105 are shown.
(2) Method of Simultaneously Transmitting Multidirectional Beams
FIG. 6 is a schematic diagram for explanation of transmission and reception of ultrasonic beams according to another conventional method. In this method, plural kinds of drive signals are simultaneously supplied from plural pulsers to ultrasonic transducers 101. For example, as shown in FIG. 6, two sets of timing pulses including “A” pulse and “B” pulse are applied to one set of elements so that an ultrasonic beam “A” and an ultrasonic beam “B” are generated. In the case where the “A” pulse and the “B” pulse overlap, they form a common pulse. Thus, plural ultrasonic beams are simultaneously transmitted in plural directions (two directions in FIG. 6).
When the ultrasonic beams are received, reception beam forming processing is performed on the RF signals outputted from the ultrasonic transducers 101 in accordance with the directions in which the ultrasonic beam “A” and the ultrasonic beam “B” have been transmitted, and thereby, the two ultrasonic beams are separated. However, since sufficient separation cannot be obtained in the case where the angular difference between the transmitted ultrasonic beam “A” and ultrasonic beam “B” is small, a scheme is required for distinguishing these ultrasonic beams.
As a related technology, Japanese Patent Application Publication JP-A-7-104063 discloses an ultrasonic object measurement apparatus having a relatively simple construction and capable of precisely measuring existence or nonexistence of an object and the distance by reducing (i) influence of surrounding ultrasonic noise and (ii) influences of occurrence of mutual interference and multiple reflection waves when plural ultrasonic converters are operated close to each other in parallel (page 1, FIG. 1). This ultrasonic object measurement apparatus performs spread frequency modulation on a tone burst wave by using a pseudo-noise signal and transmit the wave, and obtains cross-correlation between the signal generated by receiving and demodulating the reflection wave and the pseudo-noise signal used for the spread frequency modulation. The existence or nonexistence of reception of reflection signal from the object is determined according to a degree of the cross-correlation, and further, the distance is measured. Furthermore, plural tone burst waves are distinguished by sending out transmission tone burst wave by using spread frequency modulation signals distinguished by sequential switching among plural pseudo-noise signals.
JP-A-7-104063 discloses that plural M-sequences (maximal-length sequences) codes having different periods from each other are used as plural pseudo-noise signals. However, there is a problem that the crosstalk among the plural tone burst waves, which are respectively modulated by using the plural M-sequences codes, does not become smaller than a certain fixed value in the worst case.
Further, International Publication WO97/36188 discloses an ultra-wideband interference radar system for optimization of radio interference discrimination ability and evaluation of Doppler shift of reflection radar echo. In this radar system, the transmission and reception process is divided into multiple and continuous sub-processes, and each sub-process includes transmission and reception of a signal having a relative bandwidth of a part of one octave, subsequent thereto. The signals received by different narrow-band transmissions are used for reconstruction of wideband radar data in accordance with the pulse compression technology. However, there is a problem that time is required for data collection because the transmission and reception process is divided into multiple and continuous subprocesses.